As is well known, an induction generator is operated on the principle that an electromotive force in proportion to a rate at which the magnetic flux crosses a coil decreases is induced in a coil (Faraday's law of electromagnetic induction). According to Lenz's law, an induced electromotive force is generated in a direction in which a current that acts against a change in the magnetic flux is generated.
For example, as shown in FIGS. 1A and 1B, assuming that the magnetic flux .phi. crossing a circular coil 1 at a perpendicular direction moves in the A.fwdarw.B direction as indicated by the arrow, a current I.sub.1 flows in accordance with Faraday's law of electromagnetic induction so that the pointer of a galvanometer 2 swings clockwise (+ direction) and then returns to the zero position. When the magnetic flux .phi. moves in the B.fwdarw.C direction, a current I.sub.2 flows so that the indicator of the galvanometer 2 swings counterclockwise (- direction) and then returns to the zero position.
Generally, an induction generator is constructed such that an electromotive force is induced according to Flemming's right-hand rule by a conductor cutting magnetic flux lines (FIG. 1A) or by the magnetic flux lines crossing the conductor (FIG. 1B).
A rotor in an induction generator is usually embodied by a one-piece body having alternately disposed N-poles and S-poles. When there are two magnetic poles, the N-pole and the S-pole are opposite to each other. When there are more than two magnetic poles (for example, four magnetic poles or six magnetic poles etc.), the N-pole and the S-pole alternate, resulting in a N-S-N-S- . . . succession.
In this background, a unipolar induction generator is a special case wherein an electromotive force is generated by a conductor cutting the magnetic flux while moving or rotating, and a direct current is supplied through a slip ring. In other words, a unipolar induction motor is unique in its construction characterized by a non-alternating magnetic field traveling in the same direction.
In the conventional induction generator such as the one described above, improvement in energy conversion efficiency is attained such that the rotor is constructed of a ferrite or rare-earth magnet characterized by a high energy product and a small reversing permeability (recoil permeability). Alternatively, the extent of demagnetization due to generation of a counter magnetic field in an induction coil is reduced allowing the single polarity of the rotor to interact with the stator in forming a magnetic circuit. However, despite these measures, reduction in energy conversion efficiency due to a counter magnetic field of the rotor core, more specifically, due to demagnetization resulting from the counter magnetic field caused by armature reaction presents a serious problem.
The present invention has been developed in view of the above points, and its object is to provide an induction generator having a pair of magnetic poles of the same polarity opposed to each other with respect to a rotation shaft, wherein a high energy conversion efficiency is attained.